1.Field of the Invention
The present invention relates to a viscous coupling for a vehicle and the like, and more particularly to a viscous coupling provided with an operating chamber filled with a viscous fluid, the viscous resistance of which to rotation of a plurality of resistance plates of torque-transmitting members makes it possible to transmit torque from one of the torque-transmitting members to the other thereof.
2. Description of the Prior Art
As is well known, a conventional viscous coupling is constructed of: a first torque-transmitting member; a second torque-transmitting member rotatably arranged relative to the first torque-transmitting member; an operating chamber which is hermetically defined by a housing member and a shaft member while filled with a viscous fluid such as high viscosity silicone oils; a plurality of first resistance plates so mounted on the first torque-transmitting member as to be non-rotatable relative to the first torque-transmitting member; and a plurality of second resistance plates so mounted on the second torque-transmitting member as to be non-rotatable relative to the second torque-transmitting member, the second resistance plates being spaced alternately with the first resistance plates in the operating chamber; whereby the viscous resistance of the viscous fluid to rotation of the first ones or second ones of the resistance plates to the other (i.e., the second ones or first ones) of the resistance plates provides a means for transmitting torque between the first resistance plates and the second resistance plates, which torque so automatically varies as to be substantially proportional to a difference in rotational speed between the first resistance plates and the second resistance plates of the torque-transmitting members, for example, one of which torque-transmitting members is connected with a front-wheel shaft while the other of which torque-transmitting members is connected with a rear-wheel shaft.
The conventional viscous coupling of this type is disclosed in, for example such as Japanese Patent Laid-Open No. 50349/1983 (in Japanese, called Tokkaisho 58-50349), and employed as: a part of a power-transmitting unit interposed between a front-wheel axle and a rear-wheel axle of a four-wheel drive vehicle; or a limited slip differential interposed between opposite wheels of a front-wheel axle of a front-wheel drive vehicle and the like.
In operation, for example, when rear wheels of the four-wheel drive vehicle provided with such conventional viscous coupling are stuck in the mud and the like to produce a difference in rotational speed between the front-wheel axle and the rear-wheel axle of the four-wheel drive vehicle, the viscous coupling of the four-wheel drive vehicle limits the difference in rotational speed between the front-wheel axle and the rear-wheel axle to enable the vehicle to escape from the mud and the like. More particularly, in case that the rear-wheel axle of the vehicle rotates at a high speed when the rear wheels of the vehicle are stuck in the mud and the like, the temperature of the viscous fluid such as high viscosity silicone oils filled in the operating chamber of the viscous coupling increases to cause the pressure of the viscous fluid hermetically confined within the operating chamber of the viscous coupling to increase. Under such circumstances, when the pressure of the viscous fluid confined within the operating chamber of the viscous coupling reaches a predetermined value, the first resistance plates of the first torque-transmitting member are brought into a close contact with the second resistance plates of the second torque-transmitting member so that torque to be transmitted between these torque-transmitting members is abruptly increased to produce a so-called "hump torque" which enables the vehicle to escape from the mud and the like.
In case that such conventional viscous coupling is employed as a limited slip differential which is interposed between opposite wheels of a front-wheel axle of the vehicle, since the above-mentioned hump torque substantially inhibits the front wheels to produce a difference in rotational speed therebetween, it is very difficult for a driver of such vehicle to perform the steering operation of the vehicle. This is a disadvantage inherent in the conventional viscous coupling.
Consequently, in case that the viscous coupling is employed as the limited slip differential interposed between the opposite wheels of the front-wheel axle of the vehicle, it is desirable to prevent the hump torque from being produced in such viscous coupling by employing a plurality of spacer rings each of which is interposed between adjacent ones of: the first resistance plates of the first torque-transmitting member; and the second resistance plates of the second torque-transmitting member; so that the first resistance plates are prevented from being brought into a close contact with the second esistance plates, whereby the hump torque is eliminated in the thus modified conventional viscous coupling. However, such spacer rings considerably increase the number of parts required for such modified conventional viscous coupling. This is a disadvantage inherent in the modified conventional viscous coupling.